Air traffic control radars do not generally cover all the elevational angles lying between the direction of the horizon and the zenith. The non-covered zone above the radar antenna's phase center forms a cone that is called the silence cone. This silence cone can affect several air traffic control radar functions.
“En-route” radars are characterized by a very long range in the direction of their maximum radiation. Their installation at high-altitude sites, by freeing them from obstacles of the relief, guarantees their ability to exploit this low-elevation range capability. For these radars, the silence cone may be deemed too large (for example, the cruising altitude of commercial flights (FL 330) corresponds to an elevation angle of 25° at 25 km). The silence cone may also pose problems for an airport radar. Indeed, in both cases the silence cone induces gaps:
of long duration for flights at high altitude (En-route configuration);
of not such long duration but for medium altitude maneuvering flights (Airport configuration).
Conventionally, in principle the antennas of air traffic control radars, also called ATC radars, are therefore antennas of LVA (Large Vertical Aperture) type having four objectives:
a large maximum gain for “En-route” uses (long-range), typically 27 dB between 5° and 10° of elevation;
an azimuthal slender main beam (conventionally 2.5° corresponding to an antenna width of 8 m) to ensure precision and limit the effect of pollution.
a large drop to the ground to protect itself in airport configuration from the reflections of nearby buildings, such as towers of a few tens of meters, with for example a drop in gain of 2 dB per degree for elevations from 0° to −10°;
finally a transmitted or received level with the targets that is quasi-constant for long-haul flights (stable in altitude) conventionally of 5° to 40.
The antennas of the civil air traffic control (ATC) sector exhibit a cosecant squared radiation pattern, on account of their adaptation to aerial surveillance: such a pattern makes it possible to distribute in the vertical plane the energy radiated in a single exploration of the azimuthal quantum. This radiation pattern makes it possible to obtain a received signal of relatively constant amplitude for a target describing a constant-altitude trajectory. FIG. 1a illustrates the typical ATC Radar Coverage due to the gain of a conventional ATC antenna 100. More particularly, it illustrates by a curve 10 the antenna gain parametrized by angle of elevation and projected into a distance-altitude diagram.
For a cosecant squared pattern such as this and in a zone traversed according to a constant-altitude trajectory, the antenna gain G varies substantially as the square of the cosecant of the angle of elevation β, i.e. G(β)≈cosec2β, that is to say that the variation of this gain compensates the closing-in effect so as to preserve a constant received signal level over this part of the trajectory. Moreover, it is not useful to perform surveillance of the airspace at an altitude greater than the aircraft flight ceiling.
In practice, the silence cone 20 is envisaged rather as a degree of freedom for the design of the antenna. In particular the requirements would pertain rather to a guaranteed fading beyond about 50° of elevation. FIG. 2c presents the elevational patterns for the sum pathway 21 and the control pathway 22 (each relative with respect to their respective maximum) of a conventional LVA antenna. This figure shows that the antenna gain plummets at high elevation (angles beyond 50°).
The current antennas used in the ATC world, such as antenna illustrated by FIGS. 2a and 2b, comprise an antenna front panel 1, oriented forwards, insuring the sum radiation pattern 21, the front radiation pattern assigned to the control function (control pattern or “cont” pattern 22) and the difference radiation pattern, as well as a rear radiating element 12 oriented backwards, ensuring the complementary back radiation pattern assigned to the control function (control pattern 22). The outputs of such an antenna corresponding to the SUM pattern, the difference pattern and the control pattern are delivered to a standard revolving joint 2 possessing three RF wafers for the three pathways corresponding to the three patterns: the sum pathway Σ, the difference pathway Δ and the control pathway Ω. As shown in FIG. 2c, in particular, such antennas are therefore clearly not made to deal with targets in the silence cone.
Consequently, the system level solution for alleviating this state of affairs, which is common to ATC radars, consists in using dual radar coverage. These 2 radars being fairly close together make it possible to each ensure detection in the silence cone of the other.